The present invention relates generally to dewars for high temperature superconducting (HTS) filter systems for use in, for example, cellular PCS systems and, more particularly, an evacuation port and closure for such dewars.
Recently, substantial attention has been devoted to the development of high temperature superconducting radio frequency (RF) filters for use in, for example, cellular telecommunications systems. Those skilled in the art will appreciate that, when multiple HTS filters are deployed, for example, within a dewar cooled by a cryocooler, on a telecommunications tower, substantial durability and reliability issues may arise. For example, when a system is to be mounted at the top of a tower, the system must be able to withstand significant changes in climate and weather, and the system must be reliable and require minimal maintenance.
In this regard, the final step in manufacturing a durable, long life dewar, i.e., a dewar having a life span greater than 10 years, is to vacuum bake the dewar at as high a temperature as possible to degas the dewar and its components, which include temperature sensors, HTSC RF filters, getters, etc., without damaging these components and impacting their functional capability. While the dewar is baked, it is attached to a vacuum pump via a tip-off tube and evacuated. The vacuum pump will reduce the pressure within the dewar to less than 10xe2x88x924 torr and typically to less than 10xe2x88x928 torr at the time the tip-off tube is pinched off to seal the dewar. At these low pressures, the gas molecules that are outgassing from the dewar and its components will move in straight lines until the gas molecules strike a wall of the dewar or component, or another gas molecule. The gas molecules will be removed or evacuated from the dewar as they find the inside of the tip-off tube. Because the tip-off tube typically has a relative small inside diameter to minimize the size or footprint of the dewar, the degassing process tends to be quite time consuming. Typically, the dewar is vacuum baked for several days until the outgassing decreases to an acceptable level.
With the increased demand from the cellular telecommunications industry for these dewar deployed HTS filters, dewar manufacturers must find ways to increase the supply of these dewars at lower costs. Because the vacuum baking of the dewars is the most time intensive step of the manufacturing process, one option to increase the output of dewars would be to invest in more automated vacuum bakeout equipment. However, automated vacuum bakeout equipment is very expensive and, thus, this option is not necessarily the most desirable. Another option would be to reduce the time required to vacuum bake the dewars by increasing the rate at which the gas molecules are evacuated from the dewar. Because the gas molecules are only evacuated as they find the inside of the tip-off tube, the rate at which the gas molecules were evacuated would increase if the size of the tip-off tube were increased. However, because the length of the tip-off tube, or distance from the dewar at which the tip-off tube is pinched off, is directly proportional to the diameter of the tip-off tube, this option would result in an undesirable increase in the overall size or profile of these dewars.
Thus, it would be desirable to increase the manufacturing output of these dewar deployed HTS filters without drastically increasing a manufacturers capital equipment investment or increasing the size of the dewar.
The present invention is directed to an improved dewar for high temperature superconducting RF filter systems and process for manufacturing the same. In a particularly innovative aspect, a dewar in accordance with the present invention includes an oversized evacuation port, which may be greater in size by about a factor of ten than the size of an evacuation port of a conventional dewar, without increasing its overall size or profile. The incorporation of an oversized evacuation port is particularly advantageous from a manufacturing standpoint in that the time it takes to vacuum bake the dewar is substantially reduced. Specifically, there is a greater probability that the gas molecules being outgassed from the dewar and its components will find the inside diameter of a larger evacuation port and, thus, will be more quickly evacuated from the dewar. Moreover, a dewar in accordance with the present invention comprises a low profile cap that seals the evacuation port.
Prior to vacuum baking the dewar, a re-usable evacuation tool is coupled to the evacuation port of the dewar. The tool includes a housing, a capping tool positioned in the housing, and a side arm extending from the housing, which is attachable to a vacuum pump. The tool is advantageously bakeable up to a temperature of 100xc2x0 C. to 125xc2x0 C. Once the vacuum bakeout process is completed, the capping tool is actuated to cold weld the low profile cap to the tip-off flange on the end of the evacuation port and hermetically seals the dewar.